Disk drives are digital data storage devices that can enable users of computer systems to store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive includes a plurality of magnetic recording disks that are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads is disposed adjacent surfaces of the disks to transfer data between the disks and a host computer. The heads can be radially positioned over the disks by a rotary actuator and a closed loop servo system, and can fly proximate the surfaces of the disks upon air bearings.
A plurality of nominally concentric tracks can be defined on each disk surface. A preamp and driver circuit generates write currents that are used by the head to selectively magnetize the tracks during a data write operation and amplifies read signals detected by the head from the selective magnetization of the tracks during a data read operation. A read/write channel and interface circuit are connected to the preamp and driver circuit to transfer the data between the disks and the host computer.
The servo system can operate in two primary modes: seeking and track following. During a seek, a selected head is moved from an initial track to a destination track on the corresponding disk surface. Thereafter, the servo system enters the track following mode wherein the head is maintained over the center of the destination track until another seek is performed.
Read and write operations may be performed during track following mode. In order to reduce the occurrence of off-track writes (i.e. writing while the head is located more than a threshold distance from the center of the desired track), the servo system tracks the position of the head by means of a position error signal that is fed back to the servo system. The servo system moves the head in response to the position error signal in an attempt to minimize the position error signal.
In addition to responding to signals from the servo system, the position of the head is affected by operational shock and/or vibration of the disk drive. For example, vibration and/or shock may be transmitted to the disk drive through the frame or housing in which it is mounted. In particular, operational shock may result in large position errors over short periods of time. If the position error signal exceeds a fixed threshold level, data writing may be inhibited until the position error signal is reduced below the threshold level and/or a predetermined number of disk rotations has occurred. Some disk drives include a shock sensor that outputs a shock signal which indicates the occurrence and magnitude of a shock that the disk drive is experiencing. Although inhibiting writes based on a position error threshold and shock signal threshold is implemented in some disk drives, off-track writes may occur at an unacceptable rate if the threshold values are too high. In contrast, if the threshold values are too low, the data throughput of the disk drive may be significantly impaired.